1. Field of the Invention
This invention relates to a drawing head for a drawing machine comprising a base plate with a central bore nonrotatably mounted to a drawing machine, a rotatable straightedge support under the base plate with a central pin extending through the central bore, a suitable nonrotatable cover plate on the pin which has a gripping knob above the base plate and cover plate, and a locking device for the underside causing releasable retention of the straightedge support with respect to the spacing ring, and a locking device for the top side causing releasable retention of the spacing ring with respect to the base plate.
2. Description of the Prior Art
Such a drawing head is shown in German Patent Specification (Offenlegungschrift) No. 26 42 424.8 (Gebrauschmuster No. 76 29 427.0), in which the central bore of the base plate has a securely countersunk bearing ring and the central pin is constructed as a pivot pin. The spacing ring is nonrotatably secured with a locking disc positioned on top, which is rotatably positioned on the central pin of the straightedge support over a tightly mounted bearing ring of the base plate in order to enable turning of the spacing ring as well as enabling the spacing ring to lock the straightedge support against the base plate, through a displacement of the spacing ring on the underside.
Through the engagement of the tightly mounted bearing ring in the central bore of the base plate, the maximum inner diameter of the base plate is reduced. The reduction of the diameter must be compensated through a lengthening (in the axial direction) of the bearings in order to achieve desired high precision in the bearing support. The manufacture of this relatively long pivot pin and bearing ring results in problems in production and installation. Moreover, studies have shown that smaller, and particularly flatter, drawing heads are easier and more convenient to use. For example, the draftsman generally attempts to constantly avoid contact with the drawing board with the hand which operates the drawing head in order to enable exact operation and braking of the drawing head.
German Pat. (Patentschrift) No. 832 867 teaches the use of a drawing head which permits only one adjustment of the upper side, i.e., it does not have a rotatable spacing ring, so that only the assembly comprising the straightedge support, the pin, and the cover plate can be rotatably positioned with respect to the base plate. The pin is constructed as a pivot pin which is positioned in the central bore of the base plate, whereby the pivot pin has a slightly larger overall length than the thickness of the base plate, and the cover plate and the straightedge support overlap the base plate as thrust or abuttment discs.